Cdc2-mediated modulation of pp60c-src activity.

Following complete dephosphorylation, purified p60c-src was rephosphorylated with Csk. The Csk-phosphorylated form was isolated and found to be essentially inactive. Both the dephosphorylated p60c-src (Src A) and the inactive, phosphorylated pp60c-src (Src B) were then used to explore the regulatory role of other kinases and phosphatases. Phosphorylation by Cdc2 partially reactivated Csk-inactivated pp60c-src. This reactivation occurred in the absence of Tyr-527 dephosphorylation. Moreover, phosphorylation of Csk-treated pp60c-src by Cdc2 also facilitated complete reactivation by the protein-tyrosine phosphatase CD45 or by a synthetic phosphopeptide corresponding to the C-terminal, regulatory phosphorylation site (Tyr-527). These data indicate that the Src homology 2 domain of Csk-phosphorylated pp60c-src was more accessible for intermolecular interactions and that Tyr-527 was more readily dephosphorylated after treatment with Cdc2. In conjunction with in vivo studies, these data suggest that Cdc2 is involved in the regulation of pp60c-src during mitosis.